Drexel University researchers cautioned that weather patterns during the strike, including lower average wind speeds and higher humidity than in the period immediately before, could account for some of the rise in pollution. But they agreed that the increase in commuters using motor vehicles in the absence of public buses, subways, and trolleys likely contributed to dirtier air.

The invisible particles whose concentration rose during the strike are defined as having diameters of 2.5 micrometers or smaller — well below the width of a typical human hair. Technically known as fine particulates, they are produced by the burning of gasoline, coal, and other fossil fuels, among other sources, and can exacerbate breathing problems and heart disease both in the short and long term.

The strike by SEPTA workers who operate subways, buses, and trolleys began Nov. 1 and ended before dawn Nov. 7, spanning four weekdays. During the strike, average weekday maximum levels of the fine particulates, measured hourly, soared from 5 micrograms per cubic meter to 20 micrograms per cubic meter, the health department said.

"When you put more vehicles on the streets, you see a big increase here," health commissioner Thomas Farley said. "I think many people believe air pollution is a health problem of the past. Air pollution is still an important health problem."

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Philadelphia is currently "in attainment" of federal air-quality standards for fine particles, meaning its annual average level is below 12 micrograms. But the weekday data from the SEPTA work stoppage suggest that without public transit, the city likely would get a failing grade, Farley said. SEPTA's buses, trolleys, and subways provide 850,000 rides on a typical weekday.

Concentrations of fine particles are measured by the health department's Air Management Services division. Staffers analyzed results from five monitoring stations around the city.

During the pre-strike period, the average levels hovered around 5 micrograms per cubic centimeter. During the strike, they were at least a few micrograms higher throughout the day — and especially during the morning rush with the 20-microgram spike.

But weather may have contributed to the higher pollution concentrations, said Shannon Capps, an assistant professor in Drexel's department of civil, architectural, and environmental engineering.

Average morning wind speeds during the strike were slightly lower than in the few days before the work stoppage, meaning the fine particles might not have been cleared as quickly, she said. Humidity, which Capps said can contribute to formation of aerosols, also was somewhat higher during the strike period.

Independent data analyzed by Pennsylvania State University also suggest that weather played a role. A researcher at the department of meteorology and atmospheric sciences found increased concentrations of fine particulates during the same four weekdays in Baltimore and Washington.

Another weather wrinkle: While there was a spike in pollution concentrations during the morning, there was no corresponding increase for the evening rush. Capps said that could be because the air was somewhat warmer in the afternoons during the strike, leading to a thicker "boundary layer" — the area close to the earth's surface in which lots of mixing occurs.

A larger, thicker layer of air means the pollution is diluted.

"It's like putting the same amount of food dye into a shallow dish of water vs. a deeper dish of water," Capps said.

Though the pollution levels did not put the city afoul of federal clean-air standards, it is cause for concern, said Joseph Otis Minott, executive director and chief counsel of the Clean Air Council, a nonprofit headquartered in Philadelphia.

"It's not good news," Minott said. "It shows clearly that SEPTA plays just a critically important role in keeping our air quality cleaner in the region, and that we need to redouble our efforts in terms of providing alternatives to the automobile."